Because doping of materials by ion beam implantation is unique in utilizing fine scale electrostatic processes, as distinct from large scale diffusion or thermochemical processes, the problem of monitoring implantation uniformities is complex and demanding. The importance of the spatial structure of the dose, as affected by the spatial position and lateral profile of the beam, for accurate yield verification and characterization is unquestionable. VLSI process engineers concerned with yield must certainly concern themselves with fine scale dose nonuniformities, because of the fine dimensions and narrow dose tolerances of the devices being fabricated. Crystal beam scanners of varying sophistication offer hope for some of the most severe problems encountered, but they cannot be designed to insure against their own failures and must be periodically monitored in the production environment.
Other crude monitoring systems of the prior art include Faraday cups and some radiation-sensitive foil such as Mylar. Both methods have serious drawbacks. Faraday cups have a very limited spatial resolution, and it is usually impossible to obtain reliable information on the lateral extension of the ion beam. Radiation sensitive hydrocarbon foils lead to outgassing and contamination of the target chamber. They cannot be used in high current systems as thermal decomposition will destroy the foils.
At the lower limits of dose and energy, typical beam diameters shrink to a few millimeters or less, aggravating the already difficult problems of design of uniform scan systems. Therefore, high resolution measurement techniques free from other process steps must be used. Optical dosimetry has the unique capability of resolving these structural dose non-uniformity patterns in a simple-to-use real-time measurement.
The best prior art techniques used the darkening (i.e., reduced transmission to visible light) of an epitaxial silicon-on-sapphire film after ion implantation to measure implanted dose. Using this technique, the minimum doses one could measure were quite high, &gt;5.times.10.sup.12 cm.sup.-2.